This invention relates to water dispersed polymeric compositions and processes particularly useful in surface coating compositions, and more particularly pertains to aqueous polymeric blends of certain water dispersed epoxy-acrylic copolymers and epoxy phosphate ester. The polymeric blend is especially useful as a binder system for can coating compositions for the interior of steel cans used as food and beverage containers and similar sanitary coating applications.
Epoxy resins are particularly desirable for use in surface coating materials as a vehicle or polymeric binder for pigments, fillers, and other additives whereby epoxy resins advantageously provide toughness, flexibility, adhesion, and chemical resistance and water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity wherein such sanitary can coatings must not alter the product taste of canned beverages. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by adsorption of flavor by the coating, or sometimes by chemical reaction, or by some combination thereof. In commonly assigned U.S. Pat. No. 4,212,781, a process is disclosed for modifying epoxy resin by reacting the epoxy resin with addition polymerizable monomer in the presence of at least 3% by weight of benzoyl peroxide (or the free radical initiating equivalent thereof) based on monomer at a suitable reaction temperature. This reaction produces a reaction mixture containing an in-situ formed blend of resinous materials comprising an epoxy-acrylic copolymer mixture containing epoxy resin, graft epoxy-acrylic polymer, and associatively-formed ungrafted addition polymer. The in-situ polymerized monomers include acid functional monomers to provide acid functionality in the reaction mixture sufficiently high to effect stable dispersion of the resulting reaction product in a basic aqueous medium. In a preferred embodiment of U.S. Pat. No. 4,212,781, a polyglycidyl ether of bisphenol-A is reacted with a mixture of addition polymerizable monomers containing an acrylic acid such as methacrylic acid. The epoxy resin has a molecular weight above 4,000 and provides from 50% to 90% of the initial reaction mixture. The reaction takes place in the presence of benzoyl peroxide at an elevated temperature above 80.degree. C. and preferably between about 110.degree. C. and 130.degree. C. to effect addition polymerization of the monomer and product addition polymer grafted to the epoxy resin. The reaction product can be dispersed in a basic aqueous medium to form a water-reducible epoxy-acrylic copolymer mixture.
Epoxy phosphates are shown as additives to coating compositions in U.S. Pat. No. 3,686,371 and U.S. Pat. No. 4,059,550. Epoxy phosphates are disclosed as the essential binder component in U.S. Pat. No. 4,289,812, U.S. Pat. No. 4,316,922, U.S. Pat. No. 4,397,970 and U.S. Pat. No. 4,164,487.
It now has been found that certain aqueous blends of epoxy-acrylic copolymer and epoxy phosphate ester provide an excellent polymeric binder composition suitable for use as an interior coating for beverage and food containers, and especially useful for steel cans. In the beverage industry, soft drinks containing acid ingredients are particularly harsh on coatings for interiors for can, especially steel cans. Steel cans containing soft drinks are inadequately protected by either the epoxy-acrylic copolymer alone or the epoxy phosphate alone. Unexpectedly, the epoxy-acrylic copolymer in combination with epoxy phosphate ester advantageously provides improved corrosion resistance to steel cans when applied to the interior surface and subsequently used for containing acidic soft drinks. Improved solvent resistance and additional favorable characteristics of the coating composition. These and other advantages will become more apparent by referring to the detailed description of the invention and the accompanying illustrative examples.